Add !$ OMP critical around file opening for VTK

We were having problems with the `!OMP` directives around high resolution file reading in AWAE.f90. Since the file reading starts with a call to `GetNewUnit` before starting the opening, parallel calls to `GetNewUnit` could result in the same unit number handed out to two processes.  The first process would open the file and start reading, but then the second process would open a different file with the same unit number causing read errors for both processes as they attempted to read the same file at the same time.

Adding `$OMP critical` around the `GetNewUnit` and following `OpenFile...` calls so that in theory these cannot be done in parallel.

Co-authored-by: Derek Slaughter <deslaughter@gmail.com>

modules/nwtc-library/src/VTK.f90

@@ -158,9 +158,11 @@ SUBROUTINE ReadVTK_SP_info( FileName, descr, dims, origin, gridSpacing, vecLabel
          closeOnReturn = .FALSE.
       END IF
 
+      !$OMP critical
       CALL GetNewUnit( Un, ErrStat, ErrMsg )      
       CALL OpenFInpFile ( Un, TRIM(FileName), ErrStat, ErrMsg )
          if (ErrStat >= AbortErrLev) return
+      !$OMP end critical
 
        CALL ReadCom( Un, FileName, 'File header: Module Version (line 1)', ErrStat2, ErrMsg2, 0 )
          CALL SetErrStat( ErrStat2, ErrMsg2, ErrStat, ErrMsg, RoutineName )
@@ -358,9 +360,11 @@ SUBROUTINE WrVTK_SP_header( FileName, descr, Un, ErrStat, ErrMsg )
       INTEGER(IntKi)  , INTENT(  OUT)        :: ErrStat              !< error level/status of OpenFOutFile operation
       CHARACTER(*)    , INTENT(  OUT)        :: ErrMsg               !< message when error occurs
 
+      !$OMP critical
       CALL GetNewUnit( Un, ErrStat, ErrMsg )      
       CALL OpenFOutFile ( Un, TRIM(FileName), ErrStat, ErrMsg )
          if (ErrStat >= AbortErrLev) return
+      !$OMP end critical
 
       WRITE(Un,'(A)')  '# vtk DataFile Version 3.0'
       WRITE(Un,'(A)')  trim(descr)